That the incandescent lamp shines is because of heat. On the contrary, the lighting mechanism of light-emitting diode (LED) is due to the recombination of the electron-hole pair. Therefore, LED is called the cold light source.
Further, LED has the advantages of high durability, long life, light weight, and low power consumption; therefore, LED is considered as a new light source of next generation LED has gradually replaced traditional lightings to date. LED has widely used in various fields such as traffic signal, backlight module, street lighting, and medical equipment.
In the application of general lighting, the sunlight-like spectrum (white light) emitted from LED is highly appreciated in terms of human biological adaptabilities. The white light described above can be generated by mixing the three primary colors of red, blue, and green emitted from LEDs in a specific ratio through different operating currents from carefully-design circuit drivers. Because not only the cost of circuit drivers is high but also the circuit design is complicated, the method is not well-adopted. Another method employs ultraviolet light-emitting diode (UV-LED) to excite red, blue, and green phosphors capable of absorbing a part of light emitted by UV-LED and emitting the red color light, the blue color light, and the green color light. The red color light, the blue color light, and the green color light are mixed to generate the white light. Nevertheless, the luminous efficiency of UV-LED still needs to be improved so the application of the product is not widespread.
When the electric current drives the LED, in addition to the electric energy-photo energy conversion mechanism, part of the electric energy is transformed into the thermal energy, thus causing changes in the photoelectric characteristics. FIG. 1 illustrates the curve of the photoelectric characteristics of blue light LED and red light LED when the junction temperature (Tj) of the LED is increased from 20° C. to 80° C. As shown in FIG. 1, the vertical axis represents the relative value of the photoelectric characteristic value at different junction temperatures compared with that at 20° C. junction temperature of the light emitting device, such as light output power (P0; rhombus symbol), wavelength shift (Wd; triangle symbol), and forward voltage (Vf; square symbol). The solid line shown in FIG. 1 represents the characteristic curve of the blue light LED, and the dotted line shown in FIG. 1 represents the characteristic curve of the red light LED. When the junction temperature is increased from 20° C. to 80° C., the light output power of the blue light LED drops about 12% and the hot/cold factor is about 0.88; the light output power of the red light LED drops about 37% and the hot/cold factor is about 0.63. Furthermore, in terms of the wavelength shift, there is no big difference between the blue light LED and the red light LED but is only slightly changed with the difference of Tj. In terms of the forward voltage changes, when the junction temperature is increased from 20° C. to 80° C., the decline of the blue light LED and the red light LED is respectively about 7˜8%. Namely, the equivalent resistances of the blue light LED and the red light LED decline about 7˜8% under the operation of constant current. As mentioned above, because the temperature dependences of the blue light LED and the red light LED photoelectric characteristics are different, the undesirable phenomenon of the unstable red/blue light output power ratio happens during the period from the initial operation to the steady state. When the Warm white light-emitting device comprising the red light LED and the blue light LED is used in the lighting field, the light color instability during the initial state and the steady state owing to the different hot/cold factors of the blue light LED and the red light LED causes the inconvenient when using the lighting.